49ers CB Richard Sherman will miss preseason opener

Given his age and medical file, it wasn’t clear how much 49ers cornerback Richard Sherman was going to play in the preseason opener Thursday against the Cowboys.

But now it’s clear that Sherman won’t be playing at all.

On Sunday, head coach Kyle Shanahan said the hamstring injury Sherman sustained in practice Friday was more serious than initially believed and will keep him sidelined for at least a week. Shanahan said Sherman had a Grade 1 strain.

“It’s a little higher (in severity) than we expected,” Shanahan said, “so we’ve got to be careful with that.”

Sherman, 30, who missed the offseason program due to a torn Achilles he sustained in November, has participated in six of the first nine training-camp practices. He had been given two rest days before he was sidelined midway through Friday’s session. Jimmie Ward will assume his spot in the starting lineup.

RG’s return not imminent: A year after he was sidelined with a serious knee injury in training camp, it appears the same story could be unfolding for right guard Joshua Garnett.

On Sunday, after Garnett missed his sixth straight practice, Shanahan said Garnett would miss at least another week with an injury to the same knee that required surgery last year. Garnett, a 2016 first-round pick from Stanford, has not played since his rookie year. Last week, Shanahan had indicated Garnett would be returning to practice soon.

“His knee’s bothering him,” Shanahan said. “He’s gotten some other opinions. We’re going to give him a week, take this week off, continue to get treatment and then reassess it next week.”

With Garnett out, Mike Person has taken most of the first-team snaps at right guard, where Erik Magnuson has also played with the starters. Jonathan Cooper, a candidate to start at the position, participated in team drills Sunday on a limited basis for the first time after undergoing offseason knee surgery. Cooper played with the second- and third-string groups Sunday.

On Sunday, undrafted rookie guard Alan Knott was excused from practice due to the death of his father.

The 49ers addressed the position by signing guard Chris Gonzalez, an undrafted rookie from San Jose State, and waiving safety Corey Griffin.